add a new django-admin command to dump out a course structure document. ...

...This is response to an emergency request from Harvard researchers.

NOTE: This was written for Jim Waldo as he needed a mapping table from the i4x://.... identifiers to some contextual information (e.g. where is that object in relation to the whole tree, what is the metadata, etc.). Jim Waldo signed off on the utility of it so I'd like to get this on master so that it can be part of the weekly analytics dump that is provided for Harvard.

NOTE: There's uncertainty as to whether this dump really fulfills the requirements or not. Keeping PR open for discussion.